The present application relates to a lens sheet which is preferable for use as, for example, a transmission type screen for a rear projection television set, to a transmission type screen using the lens sheet, and to a rear projection type display using the transmission type screen.
In recent years, there has been a rapid spread of rear projection television sets (rear projection type displays) in which picture light modulated by a liquid crystal display, a digital mirror device or the like is displayed by projecting onto a transmission type screen while being enlarged by projection means such as a projection lens. In use of the rear projection television set, the picture light is transmitted through a screen to form an image in the vicinity of the front surface of the screen, and the image-forming picture light is recognized by a viewer from the front side of the system.
The transmission type screen for the rear projection television set has two sheet-like members which are disposed proximate to each other, with an appropriate spacing therebetween. One of the sheet-like members is a lenticular lens sheet having a lens array on the rear side thereof, and the other is a Fresnel lens which is formed with a Fresnel lens on the front side thereof and disposed to face the rear side (lens-formed surface) of the lenticular lens sheet.
The lenticular lens sheet has a structure in which a lens layer formed of a UV-curing type resin is laminated on a transparent plastic base of polyethylene terephthalate or the like. On the other hand, the Fresnel lens sheet is generally composed of a glass or an injection molded body of a transparent synthetic resin material such as an acrylic resin, a polycarbonate resin, etc.
Meanwhile, in the rear projection television screen, if the spacing between the two sheet-like members is too large, the formed image would be blurred to be a double image, and the picture would be distorted or become obscure. Therefore, it is ideal that the lenses of the sheet-like members are in contact with each other at central portions thereof.
In view of this, one or both of the sheet-like members, for example the lenticular lens sheet is preliminarily formed in a cambered state. The lenticular lens sheet in the cambered state is laid on the Fresnel lens sheet serving as the other sheet-like member, then a pressure is so exerted that the peripheral edge portions of the lenticular lens sheet is laid just on the peripheral edge portions of the Fresnel lens sheet, and the peripheral edge portions of both the sheets are fixed. With the peripheral edge portions of both the sheet-like members fixed integrally, the spacing between the lenticular lens sheet and the Fresnel lens sheet can be prevented from being too large.
However, the pressing of the lens-formed surfaces of the lenticular lens sheet and the Fresnel lens sheet against each other may lead to deformation of the lens elements on the lenticular lens sheet side. The deformation of the lens elements, if occurred, would lead to a change or changes in optical characteristics, whereby the optical path of the outgoing light may be changed, the image formed may be blurred, and the picture may be distorted or become obscure.
Besides, even in the case where the contact pressure between the lens elements is not so strong as to cause deformation of the lenticular lens sheet, upon a situation where vibration is continuingly applied to the system, as for example during transportation, the induced contact between the lens elements may cause friction and marring on the surface of the lenticular lens sheet, and the image displayed may be turned into a double or triple image due to the mars.
In order to solve such a problem, there have been made various proposals as to improvements in the resin composition constituting the lenticular lens sheet. It has been found, however, that the performances obtained according to the proposals are insufficient for the performance required of the fly-eye lens (one of lenticular lenses) attracting attention in recent years.
To be more specific, Japanese Patent Laid-open No. 2005-49795 describes a lens sheet composed of a thermoplastic resin wherein universal hardness, compressive modulus, and dissipation factor are specified. However, since a thermoplastic resin is used, this configuration is unsatisfactory from the viewpoint of precise shape reproduction and thermal strain. Besides, in Japanese Patent Laid-open No. 2003-313445 and JP-A-2005-503578, a resin composition for an optical element is disclosed which has a specified creep deformation coefficient and a specified compressive modulus. However, these indices have been unsatisfactory in terms of correspondence with the deformation in the case where a low level of pressure is exerted for a long period of time.
On the other hand, Japanese Patent Laid-open No. 2003-84101 discloses a technique of determining the relationship between the dissipation factor of a resin composition and temperature and specifying a range of peak width. However, this is also insufficient in terms of correspondence with the deformation in the case where a low level of pressure is applied for a long period of time. Further, Japanese Patent Laid-open No. 2001-228549 discloses a lens sheet which has a specified dissipation factor of dynamic viscoelasticity and a specified coefficient of dynamic friction. However, these indices pertain to frictional wear, and it is impossible to define the correspondence thereof with the deformation in the case where a low level of pressure is exerted for a long period of time.
In the case of a cylindrical lens conventionally used as a lenticular lens, the portions of contact thereof with a Fresnel lens are a plurality of line form portions. On the other hand, in the case of a fly-eye lens, the portions of contact are a plurality of spot form contact portions, and the area of contact is smaller. Therefore, the pressure exerted on the lens element is greater in the case of the fly-eye lens than in the case of the cylindrical lens. Accordingly, the pressure resistance and mar resistance performances required of a fly-eye lens are higher than those required of a cylindrical lens. However, it has been found as a result of the present inventors' research that the physical properties disclosed in Patent Documents described above cannot insure long-term properties as well as short-term properties.